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        <div class="span12">
          <h4 id="title">NDVI with a Dynamic Color Ramp</h4>
          <p class="tags">
            <span class="badge-group">
              <a
                href="./index.html?q=cog" class="badge badge-info">cog</a
              ><a
                class="badge badge-info tag-modal-toggle text-white"
                data-toggle="modal"
                data-target="#tag-example-list"
                data-title="cog"
                data-content="
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-stretch.html&quot;&gt;Band Contrast Stretch&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-style.html&quot;&gt;Change Tile Layer Style&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog.html&quot;&gt;Cloud Optimized GeoTIFF (COG)&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-pyramid.html&quot;&gt;GeoTIFF tile pyramid&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-overviews.html&quot;&gt;GeoTIFF with Overviews&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-math.html&quot;&gt;NDVI from a Sentinel 2 COG&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action active&quot; href=&quot;./cog-colors.html&quot;&gt;NDVI with a Dynamic Color Ramp&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-math-multisource.html&quot;&gt;NDVI+NDWI from two 16-bit COGs&lt;/a&gt;"
                tabindex="0"
                >8</a>
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                href="./index.html?q=ndvi" class="badge badge-info">ndvi</a
              ><a
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                data-target="#tag-example-list"
                data-title="ndvi"
                data-content="
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-math.html&quot;&gt;NDVI from a Sentinel 2 COG&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action active&quot; href=&quot;./cog-colors.html&quot;&gt;NDVI with a Dynamic Color Ramp&lt;/a&gt;
                  &lt;a class=&quot;list-group-item list-group-item-action&quot; href=&quot;./cog-math-multisource.html&quot;&gt;NDVI+NDWI from two 16-bit COGs&lt;/a&gt;"
                tabindex="0"
                >3</a>
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<table>
  <tbody>
    <tr>
      <td>Min NDVI</td>
      <td><input type="range" id="min-value-input" min="-1.0" max="-0.1" step="0.01"></td>
      <td class="data" id="min-value-output"></td>
      <td><input type="color" id="min-color"></td>
    </tr>
    <tr>
      <td>Max NDVI</td>
      <td><input type="range" id="max-value-input" min="0.1" max="1.0" step="0.01"></td>
      <td class="data" id="max-value-output"></td>
      <td><input type="color" id="max-color"></td>
    </tr>
  </tbody>
</table>

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        <div class="span12">
          <p id="shortdesc">NDVI from a COG with a dynamic color ramp</p>
          <div id="docs"><p>The GeoTIFF layer in this example draws from two Sentinel 2 sources: a red band and a near infrared band. The layer style includes a <code>color</code> expression that calculates the Normalized Difference Vegetation Index (NDVI) from values in the two bands.  The <code>interpolate</code> expression is used to map NDVI values to colors.  The &quot;stop&quot; values for the color ramp are derived from application provided style variables.  Using the inputs above, the min and max colors and values can be adjusted.  The <code>layer.updateStyleVariables()</code> method is called to update the variables used in the interpolated color expression.</p>
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      <div class="row-fluid">
        <h5 class="source-heading">main.js</h5>
        <pre><code id="example-js-source" class="language-js">import 'ol/ol.css';
import GeoTIFF from &#x27;ol/source/GeoTIFF&#x27;;
import Map from &#x27;ol/Map&#x27;;
import TileLayer from &#x27;ol/layer/WebGLTile&#x27;;

const segments &#x3D; 10;

const defaultMinColor &#x3D; &#x27;#0300AD&#x27;;
const defaultMaxColor &#x3D; &#x27;#00ff00&#x27;;

const defaultMinValue &#x3D; -0.5;
const defaultMaxValue &#x3D; 0.7;

const minColorInput &#x3D; document.getElementById(&#x27;min-color&#x27;);
minColorInput.value &#x3D; defaultMinColor;

const maxColorInput &#x3D; document.getElementById(&#x27;max-color&#x27;);
maxColorInput.value &#x3D; defaultMaxColor;

const minValueOutput &#x3D; document.getElementById(&#x27;min-value-output&#x27;);
const minValueInput &#x3D; document.getElementById(&#x27;min-value-input&#x27;);
minValueInput.value &#x3D; defaultMinValue.toString();

const maxValueOutput &#x3D; document.getElementById(&#x27;max-value-output&#x27;);
const maxValueInput &#x3D; document.getElementById(&#x27;max-value-input&#x27;);
maxValueInput.value &#x3D; defaultMaxValue.toString();

function getVariables() {
  const variables &#x3D; {};

  const minColor &#x3D; minColorInput.value;
  const maxColor &#x3D; maxColorInput.value;
  const scale &#x3D; chroma.scale([minColor, maxColor]).mode(&#x27;lab&#x27;);

  const minValue &#x3D; parseFloat(minValueInput.value);
  const maxValue &#x3D; parseFloat(maxValueInput.value);
  const delta &#x3D; (maxValue - minValue) / segments;

  for (let i &#x3D; 0; i &lt;&#x3D; segments; ++i) {
    const color &#x3D; scale(i / segments).rgb();
    const value &#x3D; minValue + i * delta;
    variables[&#x60;value${i}&#x60;] &#x3D; value;
    variables[&#x60;red${i}&#x60;] &#x3D; color[0];
    variables[&#x60;green${i}&#x60;] &#x3D; color[1];
    variables[&#x60;blue${i}&#x60;] &#x3D; color[2];
  }
  return variables;
}

function colors() {
  const stops &#x3D; [];
  for (let i &#x3D; 0; i &lt;&#x3D; segments; ++i) {
    stops[i * 2] &#x3D; [&#x27;var&#x27;, &#x60;value${i}&#x60;];
    const red &#x3D; [&#x27;var&#x27;, &#x60;red${i}&#x60;];
    const green &#x3D; [&#x27;var&#x27;, &#x60;green${i}&#x60;];
    const blue &#x3D; [&#x27;var&#x27;, &#x60;blue${i}&#x60;];
    stops[i * 2 + 1] &#x3D; [&#x27;color&#x27;, red, green, blue];
  }
  return stops;
}

const ndvi &#x3D; [
  &#x27;/&#x27;,
  [&#x27;-&#x27;, [&#x27;band&#x27;, 2], [&#x27;band&#x27;, 1]],
  [&#x27;+&#x27;, [&#x27;band&#x27;, 2], [&#x27;band&#x27;, 1]],
];

const source &#x3D; new GeoTIFF({
  sources: [
    {
      // visible red, band 1 in the style expression above
      url: &#x27;https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B04.tif&#x27;,
      max: 10000,
    },
    {
      // near infrared, band 2 in the style expression above
      url: &#x27;https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/2020/S2A_36QWD_20200701_0_L2A/B08.tif&#x27;,
      max: 10000,
    },
  ],
});

const layer &#x3D; new TileLayer({
  style: {
    variables: getVariables(),
    color: [&#x27;interpolate&#x27;, [&#x27;linear&#x27;], ndvi, ...colors()],
  },
  source: source,
});

function update() {
  layer.updateStyleVariables(getVariables());
  minValueOutput.innerText &#x3D; parseFloat(minValueInput.value).toFixed(1);
  maxValueOutput.innerText &#x3D; parseFloat(maxValueInput.value).toFixed(1);
}

minColorInput.addEventListener(&#x27;input&#x27;, update);
maxColorInput.addEventListener(&#x27;input&#x27;, update);
minValueInput.addEventListener(&#x27;input&#x27;, update);
maxValueInput.addEventListener(&#x27;input&#x27;, update);
update();

const map &#x3D; new Map({
  target: &#x27;map&#x27;,
  layers: [layer],
  view: source.getView(),
});
</code></pre>
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        <h5 class="source-heading">index.html</h5>
        <pre><code id="example-html-source" class="language-markup">&lt;!DOCTYPE html&gt;
&lt;html lang="en"&gt;
  &lt;head&gt;
    &lt;meta charset="UTF-8"&gt;
    &lt;title&gt;NDVI with a Dynamic Color Ramp&lt;/title&gt;
    &lt;!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer --&gt;
    &lt;script src="https://unpkg.com/elm-pep@1.0.6/dist/elm-pep.js"&gt;&lt;/script&gt;
    &lt;!-- The lines below are only needed for old environments like Internet Explorer and Android 4.x --&gt;
    &lt;script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,TextDecoder"&gt;&lt;/script&gt;
    &lt;script src="https://cdnjs.cloudflare.com/ajax/libs/core-js/3.18.3/minified.js"&gt;&lt;/script&gt;
    &lt;script src&#x3D;&quot;https://cdnjs.cloudflare.com/ajax/libs/chroma-js/2.1.2/chroma.min.js&quot;&gt;&lt;/script&gt;
    &lt;style&gt;
      .map {
        width: 100%;
        height:400px;
      }
      .data {
        text-align: right;
        font-family: monospace;
      }
      td {
        padding-right: 10px;
      }
      input[type&#x3D;&quot;range&quot;] {
        vertical-align: text-bottom;
      }    &lt;/style&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;div id&#x3D;&quot;map&quot; class&#x3D;&quot;map&quot;&gt;&lt;/div&gt;
    &lt;table&gt;
      &lt;tbody&gt;
        &lt;tr&gt;
          &lt;td&gt;Min NDVI&lt;/td&gt;
          &lt;td&gt;&lt;input type&#x3D;&quot;range&quot; id&#x3D;&quot;min-value-input&quot; min&#x3D;&quot;-1.0&quot; max&#x3D;&quot;-0.1&quot; step&#x3D;&quot;0.01&quot;&gt;&lt;/td&gt;
          &lt;td class&#x3D;&quot;data&quot; id&#x3D;&quot;min-value-output&quot;&gt;&lt;/td&gt;
          &lt;td&gt;&lt;input type&#x3D;&quot;color&quot; id&#x3D;&quot;min-color&quot;&gt;&lt;/td&gt;
        &lt;/tr&gt;
        &lt;tr&gt;
          &lt;td&gt;Max NDVI&lt;/td&gt;
          &lt;td&gt;&lt;input type&#x3D;&quot;range&quot; id&#x3D;&quot;max-value-input&quot; min&#x3D;&quot;0.1&quot; max&#x3D;&quot;1.0&quot; step&#x3D;&quot;0.01&quot;&gt;&lt;/td&gt;
          &lt;td class&#x3D;&quot;data&quot; id&#x3D;&quot;max-value-output&quot;&gt;&lt;/td&gt;
          &lt;td&gt;&lt;input type&#x3D;&quot;color&quot; id&#x3D;&quot;max-color&quot;&gt;&lt;/td&gt;
        &lt;/tr&gt;
      &lt;/tbody&gt;
    &lt;/table&gt;
    &lt;script src="main.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
&lt;/html&gt;</code></pre>
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      <div class="row-fluid">
        <h5 class="source-heading">package.json</h5>
        <pre><code id="example-pkg-source" class="language-json">{
  &quot;name&quot;: &quot;cog-colors&quot;,
  &quot;dependencies&quot;: {
    &quot;ol&quot;: &quot;6.13.0&quot;
  },
  &quot;devDependencies&quot;: {
    &quot;parcel&quot;: &quot;^2.0.0&quot;
  },
  &quot;scripts&quot;: {
    &quot;start&quot;: &quot;parcel index.html&quot;,
    &quot;build&quot;: &quot;parcel build --public-url . index.html&quot;
  }
}</code></pre>
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